An electronic circuit is often packaged in a protective casing, and such an electronic circuit may include a ECU (electronic control unit) for controlling input and output devices for electronic circuits used in general-purpose engines and electric generators.
FIG. 1 shows a typical conventional ECU package. The ECU package 10 comprises a circuit board 1, electronic component parts 2 mounted on the circuit board 1, a casing 3 receiving the circuit board 1 therein and potting material 6 filled in the casing 3 to fixedly hold the circuit board 1 in the casing 3. See Japanese patent laid open publication (kokai) No. 59-190086 (Patent Document 1), for instance. The electronic component parts 2 include large component parts 2a such as transformers and capacitors, and small component parts 2b such as surface mounted component parts and small discrete component parts.
FIG. 2 shows how such a package 10 may be fabricated. First of all, the electronic component parts 2 are mounted on the circuit board 1 (ST21). The circuit board 1 is then placed in the casing 3, and provisionally secured at prescribed positions (ST22). This can be accomplished, for instance, by passing terminals 5 extending from connectors 4 insert molded in the casing 3 into corresponding holes 7 formed in the circuit board 1 for soldering (see FIG. 1). The potting material 6 is then poured into the casing 3 so as to entirely cover the electronic component parts as well as the circuit board 1 (ST23), and is allowed to be cure or solidify in this condition (ST24).
The potting material 6 protects the electronic component parts 2 from vibrations and moisture. When no potting material is used, the large component 2a is caused to oscillate owing to external vibrations, and the resulting inertia force may cause undue mechanical stresses to the soldered parts between the large component 2a and circuit board 1. When potting material 6 is used, the circuit board 2 and large component 2a are kept immobile relative to each other even in the presence of external vibrations, and this minimizes the mechanical stresses in the soldered parts that join the large component 2a to the circuit board 1.
Furthermore, the potting material 6 seals off the circuit board 1 and electronic component parts 2 from the intrusion of moisture, and this prevents any leakage of electric current.
Patent Document 1 proposes to minimize influences of external vibrations on electronic component parts mounted on a circuit board in an outboard marine engine by orienting the major plane of the circuit board supporting the electronic component parts along the direction of the movement of the vibrations.
Meanwhile, the use of potting material is desired to be minimized because of material cost and added weight. Japanese patent laid open publication (kokai) No. 200-271002 (Patent Document 2) proposes to use two separate circuit boards including a first circuit board using potting material for the protection of component parts mounted thereon and a second circuit board using only protective coating which is effective for protecting component parts from moisture.
According to the prior art disclosed in Patent Document 1, the mounting position or orientation of the electronic component package must be properly selected, and this imposes some restriction in design. In particular, in applications where mounting space is scarce, this restriction is highly disadvantageous.
According to the prior art disclosed in Patent Document 2, the component parts mounted on the second circuit board are not protected from external vibrations. Furthermore, using two separate circuit boards causes an increase in material and manufacturing costs, and requires a larger mounting space.